Micro-lenses for CMOS imagers

ABSTRACT

A micro-lens and a method for forming the micro-lens is provided. A micro-lens includes a substrate an lens material located within the substrate, the substrate having a recessed area serving as a mold for the lens material. The recessed can be shaped such that the lens material corrects for optical aberrations. The micro-lens can be part of a micro-lens array. The recessed area can serve as a mold for lens material for the micro-lens array and can be shaped such that the micro-lens array includes arcuate, non-spherical, or non-symmetrical micro-lenses.

FIELD OF THE INVENTION

The present invention relates generally to the field of semiconductordevices and more particularly, to micro-lenses utilized in imagerdevices or displays.

BACKGROUND

The semiconductor industry currently uses different types ofsemiconductor-based imagers, such as charge coupled devices (CCDs), CMOSactive pixel sensors (APS), photodiode arrays, charge injection devicesand hybrid focal plane arrays, among others, that use micro-lenses.Semiconductor-based displays using micro-lenses are also known.

It is desirable to maximize the amount of light received by thephoto-conversion devices of an imager. One way to increase the lightreceived by the photo-conversion devices is to increase the amount oflight received by micro-lenses, which collect external light and focusit on the photo-conversion device. Another way is to enhance thepositioning of the focal point of each micro-lens to ensure that much ofthe light received by the micro-lenses is focused on thephoto-conversion devices.

Micro-lenses may be formed through an additive process in which a lensmaterial is formed on a substrate and subsequently is formed into amicro-lens shape. Micro-lenses also may be formed by a subtractiveprocess in a substrate. Known subtractive processes are complex andmanufacturing micro-lenses from such known processes is difficult.

SUMMARY

The present invention provides an easily manufactured micro-lens whichcan be used in an imager or display device. In one exemplary embodiment,the micro-lens includes a substrate and lens material located within thesubstrate, the substrate having an opening serving as a mold for thelens material. The opening can be shaped such that the lens materialcorrects for optical aberrations.

In an exemplary embodiment of an imager, the imager includes a pluralityof pixel cells each having a photo-conversion device, a mask fordirecting electromagnetic radiation to each photo-conversion device, acolor filter assembly, and a micro-lens array including a plurality ofmicro-lenses each associated with one of the pixel cells. The micro-lensarray includes a recessed area in a substrate serving as a mold for lensmaterial. The micro-lens array can be configured to effect a change infocal point between the micro-lenses to correct for optical aberrationsand/or for the wavelength dependency of the photo-conversion devices foreach of the colors detected.

In an exemplary micro-lens system embodiment, a micro-lens system isprovided that includes a first micro-lens array including a firstplurality of micro-lenses and a second micro-lens array including asecond plurality of micro-lenses. The first micro-lens array ispositioned over the second micro-lens array.

In an exemplary fabrication embodiment, a method is provided formanufacturing a micro-lens array. The method includes the acts offorming a recessed area in a substrate, wherein the recessed areaincludes a plurality of micro-lens sections having different profiles,and filling the recessed area with a lens material to form a pluralityof micro-lenses.

These and other features of the invention will be more readilyunderstood from the following detailed description of the invention,which is provided in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a portion of a lithographic mask constructed inaccordance with an exemplary embodiment of the invention.

FIGS. 2A-2F are cross-sectional views illustrating formation of amicro-lens using the lithographic mask of FIG. 1.

FIG. 2G is a top view illustrating the lens mold shown in FIG. 2D.

FIG. 3 illustrates a process for forming a micro-lens using thelithographic mask of FIG. 1.

FIG. 4 is an exploded view of an imager with a micro-lens formed fromthe lithographic mask of FIG. 1.

FIG. 5 is a view of an imager formed in accordance with anotherexemplary embodiment of the invention.

FIG. 6 is a cross-sectional view of a micro-lens system formed inaccordance with another exemplary embodiment of the invention.

FIG. 7 is a cross-sectional view of an alternative micro-lens system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 and 2A illustrate a portion of a lithographic mask 30 used informing a first exemplary embodiment of a micro-lens array. Thelithographic mask 30 as shown includes a variety of openings 32 _(A-D).The openings 32 _(A-D) are shown to be circular in shape, although itshould be understood that the openings 32 _(A-D) may take anysymmetrical or non-symmetrical shape. The openings 32 _(A-D) are furtherillustrated as being in a pattern whereby the largest opening 32 _(A) isimmediately surrounded by the next largest openings 32 _(B), with thesmaller openings 32 _(C) and 32 _(D) being positioned peripheral to theopenings 32 _(B). The illustrated lithographic mask 30 is suitable formanufacturing a plurality of micro-lenses at a time. The lithographicmask 30 may further include many similar patterns for the formation of agreater number of micro-lenses.

With specific reference to FIGS. 2A-2G and 3, next will be described aprocess for forming a first micro-lens array embodiment. An objective isto form, through controllable design and processing, a plurality ofmicro-lenses whose focal points change gradually across the micro-lensarray. The process generally includes a subtractive step followed by anadditive step, another subtractive step, and an optional out gassingstep.

At Step 100 (FIG. 3), a photo resist layer 36 is formed on a substrate40. The substrate 40 is an insulating layer of an imager pixel arraywhich is above a semiconductor substrate on which the pixel array isformed. Preferably, the substrate 40 may be formed of any well knowntransparent insulator material, for example, SiO₂, among many others.

As illustrated (FIGS. 2A, 2B), the photo resist material forming thephoto resist layer 36 is a positive resist. Nonetheless, the photoresist material forming the photo resist layer 36 may be a negativeresist. If the photo resist layer 36 is formed of a negative resistmaterial, the lithographic mask would have to be inverted from thelithographic mask 30 illustrated in FIGS. 1 and 2A.

Next, at Step 105, electromagnetic radiation, such as light 34, isdirected through the lithographic mask 30 to image the photo resistlayer 36 and expose first portions 38, leaving second portions 39unexposed. Following Step 105, at Step 110 portions of the photo resistlayer 36 are removed using a suitable resist developer, such as, forexample, dilute TMAH. The photo resist layer 36 illustrated in FIGS. 2Aand 2B is a positive resist, and so the first portions 38 are thoseportions imaged by the electromagnetic radiation 34 at Step 105 andsubsequently removed at Step 110 to form openings 42 _(A-C). It shouldbe appreciated, however, that for a photo resist layer 36 that is anegative resist, a lithographic mask inverse to the lithographic mask 30would be required and the electromagnetic radiation 34 would image thesecond portions 39 at Step 105, and the unexposed first portions 38 ofthe photo resist layer 36 would be removed at Step 110.

The remainder of the photo resist layer 36 is then used as an etch mask.Specifically, at Step 115, the substrate 40 is etched through the etchmask (remainder of photo resist layer 36) to form a recessed area 46(FIG. 2D). Etching material 44, which is at least partially if not fullyisotropic, is placed within the openings 42 _(A-C) and allowed to etchthrough both the photo resist layer 36 and the substrate 40 (FIG. 2C).The etching material 44 may be a dry etching material, a wet etchingmaterial, or a combination of wet and dry etching materials. Asillustrated in FIGS. 2C and 2D, during the etching process, holes 46_(A-C) are formed in the substrate 40, and due to the isotropic natureof the etching material and the crystalline orientation of the substrate40, the holes 46 _(A-C) grow in size and depth and eventually combine toform the recessed area 46, which is then used as a mold for themicro-lens array. The targeted shape of the recessed area 46 includes asmooth surface which is obtained by a controlled merging of theneighboring holes 46 _(A-C). The holes 46 _(A-C) are sized andpositioned in such a way to, along with the underetch properties of theetching material 44, determine the shape of the mold for the micro-lensarray.

As illustrated in FIGS. 2C-2F, the etching material 44 in the openings42 _(A-C) etches the substrate 40 to assist in the formation of theholes 46 _(A-C), which grow to form micro-lens molds 46 _(A-C).Specifically, the hole 46 _(A) grows to become the micro-lens mold 46_(A) of the recessed area 46. In likewise fashion, the etching material44 in the openings 42 _(B) etches the substrate 40 to combine all theholes 46 _(B) together and to create the micro-lens molds 46 _(B), andthe etching material 44 deposited in the openings 42 _(C) etches thesubstrate 40 to enlarge all the holes 46 _(C) to create the micro-lensmolds 46 _(C). Although the micro-lens molds 46 _(A-C) are shown to havearcuate portions, it should be appreciated that one or more of themicro-lens molds 46 _(A-C) may instead have non-spherical and evennon-symmetrical aspects. Further, it should be appreciated that more orless than three micro-lens molds may be formed, depending upon thenumber of openings in the photo resist layer 36.

Next, at step 120, the remainder of the photo resist layer 36 is removedand the micro-lens array mold 46 for the micro-lens array is filled toform an array of micro-lenses 48 having micro-lenses 48 _(A-C).Preferably, the micro-lens array mold 46 is filled with a lens material47 (FIG. 2E) having a different refractive index than the refractiveindex of the substrate 40, which, as noted, may be formed of silicondioxide. If forming a positive micro-lens, the refractive index of thelens material 47 is greater than the refractive index of the substrate40, and hence the micro-lenses thus formed are focusing lenses causinglight rays to converge. One preferred positive micro-lens includes asilicon nitride lens material 47 in a silicon dioxide substrate 40. Ifforming a negative micro-lens, the refractive index of the lens material47 is less than the refractive index of the substrate 40, and hence, themicro-lenses thus formed cause light rays to diverge. One preferrednegative micro-lens includes a low refractive index polymer, mostpreferably a transparent photosensitive polymer, lens material 47 in asilicon dioxide substrate 40.

With specific reference to FIGS. 2D-2G, the micro-lens molds 46 _(A-C)are filled with the lens material 47, to form a first micro-lens 48 _(A)surrounded by second micro-lenses 48 _(B) and third micro-lenses 48_(C). The illustrated first micro-lens 48 _(A) has a generally circularprofile. The surrounding second micro-lenses 48 _(B) each includes anarcuate portion and they completely surround the first micro-lens 48_(A). The illustrated third micro-lenses 48 _(C) each includes anarcuate lens portion separated from one another. Each of the thirdmicro-lenses 48 _(C) combines with a second micro-lens 48 _(B). Itshould be appreciated that the mask 30 can be so configured to producethe third micro-lenses 48 _(C) in such a way that they instead combinetogether like the second micro-lenses 48 _(B). Once the micro-lens arraymold 46 has been filled with the lens material 47, at Step 125 the lensmaterial 47 is planarized by, for example, chemical-mechanicalplanarization, to render it flush with the surface of the substrate 40.

As illustrated in FIG. 2F, a color filter array 50 including colorfilters 50 _(A-C) is then formed over the micro-lenses 48 thus made atStep 130 so that each pixel cell has one associated color filter of, forexample, red, green or blue filters. If the lens material 47 used toform the micro-lenses 48 is a special heat-decomposable polymer, at Step135 heat 52 can be applied to the micro-lenses 48 to cause thermaldecomposition, or outgassing, 54 to leave only residual gas in themicro-lenses 48. To facilitate outgassing, suitable small openings canbe provided in the color filter array.

Referring to FIG. 4, there is shown a portion of an imager 60, forexample, a CMOS imager, that may incorporate the micro-lens array 48,including the micro-lenses 48 _(A-C) formed through the processdescribed with reference to FIGS. 1-3. It should be appreciated thatother semiconductor-based imagers, as well as semiconductor-baseddisplays, may also incorporate the micro-lens array 48. The illustratedCMOS imager 60 includes a light shield 64 and a plurality of pixelcells, each with a respective photosensor 68 _(A-C), which may be aphoto-conversion device such as a photodiode or photogate. The CMOSimager 60, which includes the micro-lens array 48 formed in thesubstrate 40, also includes a color filter array 50 including a varietyof color filters, one for each micro-lens 48 _(A-C) and correspondingpixel cell and photosensor 68 _(A-C). Electromagnetic radiation, such aslight 62, is directed through the micro-lens array 48, and eachmicro-lens 48 _(A-C) focuses the light 62 on respective one of thephotosensors 68 _(A-C). The light shield 64 is configured to inhibitcross talk between the illustrated pixel cells. The micro-lens array 48effects a change in the focal point between the individual micro-lensesto correct for optical aberrations in the imager module or pixelasymmetries, for example, those due to specific space saving imagerdesigns.

FIG. 5 illustrates a portion of an imager 160, e.g., a CMOS imager, thatincludes a plurality of pixel cells. Each pixel cell includes arespective photosensor, such as the illustrated photosensors 168A, 169,169′, 171, 171′, 173, and 175′. Again, as with the light shield 64 (FIG.4), a light shield 164 is positioned and configured to suppress crosstalk between the illustrated pixel cells. The illustrated imager 160includes a color filter array 150. A micro-lens array 148 is includedwith a non-symmetrical micro-lens 149 and more symmetrical micro-lenses48 _(A), 48 _(B), and 48 _(C), whereas a micro-lens array 148′ isincluded with a non-spherical micro-lens 149′ and more symmetricalmicro-lenses 48 _(A) and 48 _(B), It should be appreciated that a matrixof micro-lens arrays, such as the arrays 48, 148, and 148′ can be formedadjacent to one another. Although the light shields 64, 164 areillustrated as part of the imager 60, 160, it should be appreciated thatthe light shields 64, 164 are optional and may not be required incertain circumstances.

By forming micro-lenses through the above-described subtractive method,individual micro-lens structures can be formed for each type of colorpixel cells to take advantage of the different absorption depths oflight in the substrate 40 due to the different wavelengths of lightwhich pass through the various filters. The focal point of eachmicro-lens is wavelength dependent. For example, light through a bluefilter absorbs at the surface of a silicon substrate and thus requires ashort focal length, while light through a red filter absorbs severalmicrons into the silicon substrate and thus requires a longer focallength. Thus, forming micro-lenses with structural differences dependingon the wavelength of light to be detected by a pixel cell enhances thelight received at each photosensor by controlling the position of thefocal point for each micro-lens.

Further, if a photosensor is not centered within its respective pixelcell, then the focal point of the micro-lens would be shifted along theX-Y plane. The use of non-symmetrical lens sections, such as thenon-symmetrical lens section 149 adjusts the focal point. Additionally,if the imager 160 is positioned so close to a primary imaging lens, forexample, a camera lens, that the size of the imager is comparable to thedistance between the camera lens and the imager, the incident angle oflight on each pixel cell changes significantly across the imager. Thisresults in signal intensity and color changes across the image, as wellas added pixel cell cross talk. By providing micro-lenses with changingshapes for respective lens sections, the effects caused by the closeproximity of the imager to the camera lens can be compensated for.

Micro-lenses according to embodiments of the invention described abovein connection with FIGS. 1-5 can be implemented in a lens system. A lenssystem can include a series of lenses that function together. FIGS. 6and 7 illustrate exemplary embodiments of lens systems according to theinvention. As shown in FIG. 6, a lens system 250 includes a micro-lensarray 48, which in turn includes the micro-lenses 48 _(A-C). Positionedover the micro-lens array 48 is a micro-lens array 248 formed in anadditive process. The micro-lens array 248 includes sphericalmicro-lenses and can be formed by techniques known in the art.

FIG. 7 depicts another embodiment of a lens system according to theinvention. In the lens system 350, includes micro-lens arrays 48 and48′, both of which are formed through the subtractive process describedabove in connection with FIGS. 1-5. Specifically, each of the micro-lensarrays 48′, 48 are formed in a substrate 40 and layer 41, respectively.After a first micro-lens array 48′ is formed in substrate 40, a secondlayer 41 of a suitable material, for example, a same material assubstrate 40, e.g. SiO₂, is deposited over substrate 40 and firstmicro-lens array 48′. Then, a second micro-lens array 48 is formed inlayer 41 by the same methods used to form micro-lens array 48′.

The lens systems 250, 350 are advantageous in that they allow foradditional control to be exerted over chromatic properties. Chromaticaberrations can be alleviated at least to some extent with the lenssystems 250, 350. The micro-lens arrays in the lens systems 250, 350 mayhave differing refractive indexes and/or different lens shapes to assistin correcting chromatic aberrations.,

While the invention has been described in detail in connection withexemplary embodiments known at the time, it should be readily understoodthat the invention is not limited to such disclosed embodiments. Rather,the invention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Accordingly, the invention is not to be seen as limited bythe foregoing description, but is only limited by the scope of theappended claims.

1. A micro-lens array for use in an imager, comprising: a semiconductorsubstrate positioned over an array of pixel cells, the semiconductorsubstrate having a bottom surface facing towards the pixel cells and anupper surface opposite the bottom surface; an opening in thesemiconductor substrate recessed below the upper surface of thesubstrate, the opening serving as a mold for a plurality ofmicro-lenses; and lens material located within the opening mold of thesemiconductor substrate, wherein the lens material forms the pluralityof micro-lenses, each of the micro-lenses having a respective focalpoint, wherein the focal point of at least one of the plurality ofmicro-lenses differs from the focal point of at least one other of theplurality of micro-lenses.
 2. The micro-lens array of claim 1, whereinthe opening has at least one arcuate portion.
 3. The micro-lens array ofclaim 1, wherein the opening mold is shaped such that the lens materialcorrects for optical aberrations.
 4. The micro-lens array of claim 1,wherein the semiconductor substrate comprises silicon dioxide.
 5. Themicro-lens array of claim 4, wherein the differing focal points of theplurality of micro-lenses focus light to different depths in thesemiconductor substrate.
 6. The micro-lens array of claim 1, wherein thelens material exhibits a refractive index greater than that of thesemiconductor substrate.
 7. The micro-lens array of claim 1, wherein thelens material exhibits a refractive index less than the semiconductorsubstrate.
 8. The micro-lens array of claim 1, further comprising aplurality of openings in the semiconductor substrate forming a pluralityof molds, wherein each opening mold contains lens material that forms arespective plurality of micro-lenses, each of the micro-lenses having arespective focal point, wherein the focal point of at least one of theplurality of micro-lenses differs from the focal point of at least oneother of the plurality of micro-lenses in the same opening.
 9. Themicro-lens array of claim 1, wherein the focal point of at least one ofthe plurality of micro-lenses differs from the focal point of at leastone other of the plurality of micro-lenses such that the focal points ofthe plurality of micro-lenses change gradually across the micro-lensarray.
 10. The micro-lens array of claim 1, wherein each micro-lens isrespectively associated with a photosensor of one of the array of pixelcells.
 11. The micro-lens array of claim 1, wherein at least one of theplurality of micro-lenses has a different shape than another one of theplurality of micro-lenses.
 12. The micro-lens array of claim 1, whereinat least one of the plurality of micro-lenses has a different size thananother one of the plurality of micro-lenses.
 13. The micro-lens arrayof claim 1, wherein at least one of the plurality of micro-lenses has adifferent profile than another one of the plurality of micro-lenses. 14.A micro-lens array, comprising: a semiconductor substrate positionedover an array of pixel cells, the substrate having a bottom surfacefacing towards the pixel cells and an upper surface opposite the bottomsurface, and the substrate being formed of silicon dioxide; an openingin the substrate recessed below the upper surface of the semiconductorsubstrate, the opening serving as a mold for a plurality ofmicro-lenses; and lens material located within the opening of thesemiconductor substrate, wherein the opening mold is shaped such thatthe lens material corrects for optical aberrations, and wherein the lensmaterial forms the plurality of micro-lenses, each of the micro-lenseshaving a respective focal point, wherein the focal point of at least oneof the plurality of micro-lenses differs from the focal point of atleast one other of the plurality of micro-lenses.
 15. The micro-lensarray of claim 14, wherein the opening mold is structured such that thefocal point of each micro-lens of the array is associated with a colorof light.
 16. The micro-lens array of claim 14, wherein the lensmaterial exhibits a refractive index greater than that of thesemiconductor substrate.
 17. The micro-lens array of claim 14, whereinthe lens material exhibits a refractive index less than that of thesemiconductor substrate.
 18. The micro-lens array of claim 14, whereineach micro-lens is respectively associated with a photosensor of one ofthe array of pixel cells.
 19. The micro-lens array of claim 14, furthercomprising a plurality of openings in the semiconductor substrateforming a plurality of molds, wherein each opening mold contains lensmaterial that forms a respective plurality of micro-lenses, each of themicro-lenses having a respective focal point, wherein the focal point ofat least one of the plurality of micro-lenses differs from the focalpoint of at least one other of the plurality of micro-lenses in the sameopening.
 20. The micro-lens array of claim 14, wherein the focal pointof at least one of the plurality of micro-lenses differs from the focalpoint of at least one other of the plurality of micro-lenses such thatthe focal points of the plurality of micro-lenses change graduallyacross the micro-lens array.